Reduction of noise



March 28, 1939. w. R. KOCH- i REDUCTION OF NOISE Filed March 28, 1936 5Sheets-Sheet 1 INVEN'I'OR Winfield R.Koch

I AM

hrroPN L 'iY March 28, 1939. w. R. KOCH REDUCTION OF NQISE Filed March28, 1936 5 Sheets-Sheet 2 \Q U N Q Q E m wm QM .m+ 03+ M v w IINVEN'I'OFI' Winfield R. [f0 7 h March 23, 1939. w R K H 2,151,773

REDUCTION OF M61515 Filed March 28, 193% s Sheets-Sheet s INVEN'I'ORWinfield H. Koch HT'I'ORNEY Patented Mar. 28, 1939 REDUCTION OF NOISEWinfield K. Koch, Merchantville, N. J., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application March 28,

14 Claims.

The invention relates to the reduction of the noise which tends to beproduced in radio receivers and similar apparatus by electrical impulsesextraneous tothe desired signal or control impulses. It has for itsprincipalobject the provision of an improved apparatus and method ofoperation whereby the noise and signal impulses may be separated fromone another without substantial distortion of the signal impulses.

Many noise reduction methods heretofore proposed have involved reducingthe signal output to zero for the duration of the noise impulse or haveinvolved merely limiting the amplitude of the noise impulse. Thesemethods have not been altogether satisfactory for the reason that theyeliminate a part of the signal impulse or otherwise distort the signalto such an extent that objectionable noise is produced at the receiver.In accordance with the present invention, this difiiculty is largelyeliminated by the provision of means which operate for the duration ofthe noise impulse to (1) open the signal channel, (2) maintain thesignal level substantially-constant at the receiver or (3) maintain asubstantially constant ratio of signal impulse level to noise impulselevel.

The invention will be better understood from the following descriptionwhen considered in connection with the accompanying-drawings and itsscope is indicated by the appended claims.

Referring to the drawings Fig, l is a wiring diagram of a noisereduction system including a delay network through which a pair ofcontrol vacuum tube devices are operated at successive instants of timeto control the grid bias potential applied from an oscillation generatorto a pair of channel opening vacuum tube devices, the signal level atpoints beyond the circuit opening devices being maintained sub 40stantially by means of a capacitor connected across the channel at apoint following the channel openingdevices,

Fig. 2 is a wiring diagram of a noise reduction system which differsfrom that of Fig. 1 in that an over-biased intermediate frequencyamplifier is utilized to performgthe function of the control tubes, andoscillation generator of Fig. 1,

Figs. 3 and 4 are explanatory diagrams relating to the operation of theapparatus, and

Fig. 5 is a wiring diagram of a noise reduction system which is similarto that of Fig. 1 but differs therefrom in that the grid bias potentialof the channel opening devices is made to depend on a resultantpotential, the compbnents of which are derived respectively from thedelay 1936, Serial No. 71,348 ((31. 250 2o network and from theintermediate frequency circuit for ensuring operation of the noisereduction apparatus over a somewhat greater range of noise-levels.

In the noise reduction circuit of Fig. 1, signal 5 impulses are suppliedfrom a diode biased audio amplifier through a delay network, an audioamplifier l2, and a pair of reversely connected channel opening vacuumtube devices l3 to a capacitor l4 and to the input circuit of an audio10 amplifier IS. The diode biased amplifier IIJ may i be the seconddetector of asuperheterodyne radio receiver or the like. The circuitopening tubes, iii are diode-biased. Grid control potential is suppliedto these diode-biased tubes, l3 from an oscillation generator l6 througha transformer ll, this control potential being controlled by means of apair of control tubes 18 and H! which have their input circuitsenergized from different points A and C of the delay network I I.

For normal operation, without large noise impulses, the capacitor M ischarged and discharged by the audio amplifier l2 operating through thecathode-anode electron discharge paths of the circuit control or circuitopening tubes l3.

When a noise impulse exceeding a predetermined level occurs, the controltube l8, which is normally biased beyond cutofi by the outer controlgrid and biasing means indicated in connection therewith, becomesoperative, since the '30 presence of a noise impulse causes the outercontrol grid to become more positive.

The tube l8 then becomes conducting for conveying oscillations from theoscillator l6 through its connection with the inner control grid. The 5amplified output from tube I6 is thereby applied to the output circuitof tube l8, and the oscillator energy is applied to the control devicesl3 through the coupling means IT.

The coupling means I! permits rapid build- 40 up or decay of theoscillatory currents and the diodes of the tubes 43 rectify theresulting voltage impressed on the' diodes through the transformer l? toproduce direct current biasing potential for the control grids of tubesl3. The 45 biasing potential is such that the control grids of the tubesl3 are biased beyond the cutofi value.

With this cut-off arrangement, the charge on the capacitor M will beunaffected by the amplifier l2, and. the instantaneous input voltage ofthe audio frequency amplifier l5 will be maintained constant at thevalue or level attained immediately preceding the operation of the tube58 or channel cut-off.

.When the signal arrives at the end of the delay network, it serves tooperate the tube IS in a similar manner to maintain the channel open fora short interval after the application of the signal to the amplifierl2. Otherwise, a noise impulse of appreciable magnitude might be appliedto the output amplifier l5. Due to the delay effect of the network II,the channel is opened before the noise impulse reaches the input circuit.of the audio amplifier l5.

In like manner, closing of the channel is delaye'd beyond the pointwhere the noise impulse again reaches the predetermined level, thisresult being effected by the control tube l9 which is biased beyondcutoff and is maintained open until the noise impulse has passed thepoint C of the delay network ll.

Various modifications of the noise reduction circuit of Fig. 1 will beapparent to those skilled in the art. Thus, the circuit may be renderedindependent of fiat automatic volume control action by so adjusting thecontrol tubes l8 and I9 that they operate whenever the instantaneousaudio voltage exceeds the direct current voltage resulting from thecarrier alone. Under these conditions, operation of these tubes iseffected on any impulse of a level exceeding the voltage correspondingto 100% modulation in the positive direction. Instead of the network H,suitable mechanical, acoustic or like means may be utilized to delay thesignal until the channel control tubes l2 and i3 have operated.

The'modification of Fig. 2 is similar to that of Fig.1 but differstherefrom in that the oscillation generator l6 and control tubes 18 and[9 are replacedby an over-biased intermediate frequency amplifier 20, anintermediate amplifier 2| is illustrated and the diode detector 22 isshown as enclosed in a receptacle separate from that of the first audioamplifier Ill.

With the overbiased intermediate frequency amplifier 20 connected asshown, intermediate frequency voltages greater than that correspondingto any predetermined percentage modulation cause the amplifier 20 tofunction. The resulting voltages impressed on the diodes and grids ofthe channel control tubes l3 cause them to open the channel before theaudio noise impulse reaches them. Closing of the channel is delayed fora period of time dependent-on the resistancecapacity constants betweengrids and cathodes of tubes [3.

If the signal impulse has the wave form indicated' at 25 and thesuperposed noise impulse has the wave form indicated at 26, the signalreceived at the audio amplifier I5 will have a wave form similar to thatindicated at 21. The difference between the present and prior methods ofnoise reduction is more clearly illustrated by Figs. 3 and 4. Thus Fig.3 illustrates the effect of previous limiter or noise reduction deviceson a signal impulse 25 which has noise impulses superimposed on it atpoints a, b and c and Fig. 4 illustrates the effect on the samecomposite Wave of a noise reduction constructed in accordance with thepresent invention. The upper dotted line in each case indicates theinstantaneous audio level corresponding to 100% modulation and the lowerdotted line similarly indicates the instantaneous audio level for zeromodulation.

The noise reduction circuit of Fig. 5 is similar to that of Fig. l withthe exception that a conductor 28 is connected back to the intermediatefrequency circuit for the purpose of injecting into the input circuitsof the control tubes l8 and 19 a bias potential which varies in response'on the diode resistor connecting to capacitor [4,

to permit a balancing action to be obtained, for voltagesoccurringacross this resistor during impulses, which cause currents to flowthrough the stray capacity to ground of the secondary winding oftransformer l1, and back through capacitor l4, thus introducing a smalldisturbance in the voltage across this capacitor. Otherwise, thesystem-of Fig. 5 operates in a manner similar to that of Fig. 1.

I claim:

l. The combination of a signal channel including a channel opencircuiting device and an amplifier in the order named, and meansincluding a capacitor connected in the input circuit of saidamplifier'for maintaining the signal in said channel following saiddevice and preceding said amplifier at a levelattained immediatelypreceding the opening of the channel by said device.

2. The combination of a signal channel including a delay network throughwhich signals are conveyed, a circuit opening device and an amplifier inthe order named, means responsive to the arrival of a relatively highlevel signal at differentpoints along said network for controlling theoperation of said device, and means interposed between saiddevice andsaid amplifier for maintaining the signal at said amplifier at a levelattained'immediately preceding the opening of the channel by saiddevice.

3. The combination of a signal channel including -a delay network and achannel opening device,"means connected to one point of said network forcausing said device to open said channel when arelatively high levelimpulse arrives at said point, and means connected to a subsequent pointof said network'for maintaining said channel'open until said impulse haspassed said subsequent point.

'4. The combination of a signal channel including a delay network and achannel opening device, means connected to one point of said network forcausing said device to open said channel when a relatively high levelimpulse arrives at said point, and means connected to a subsequent pointof said network for maintaining said channel open until said impulse haspassed said subsequent point, and means for maintaining in the channelsection following said device a predetermined signal level while saidchannel is open.

5. The combination of a delay network, a leading amplifier coupled to anintermediate point of said network, a following amplifier, a circuitopening device interposed between said amplifiers, and means connectedat network points preceding and following said intermediate point foroperating said device to maintain said device open while a relativelyhigh level impulse passes between said leading and following points.

6. The combination of a delay network, a leading amplifier coupled to anintermediate point of said network, a following amplifier, a circuitopening device interposed between said amplifiers, means-connected atnetwork points preceding and following said intermediate point foroperating said device to maintain said device open While a relativelyhigh level impulse passes between said leading and following points, andmeans interposed between said device and said followin amplifier formaintaining a predetermined impulse level at said following amplifierwhile said device is open.

7. The combination of a signal channel includinga delay network, anamplifier and a pair of reversely connected vacuum tubes interposedbetween an intermediate point of said network and said amplifier, meansincluding an oscillation generator for applying input control potentialto said tubes, and a pair of vacuum tubes energized from the oppositeends of said network for permitting the application of said controlpotential tosaid reversely connected tubes only while a relatively highlevel impulse passes from' one end of said network to the other.

8. The combination of a signal channel including a delay network, anamplifier and a pair of reversely connected vacuum tubes interposedbetween an intermediate point of said network and said amplifier, meansincluding an oscillation generator for applying input control potentialto said tubes, and a pair of vacuum tubes energized from the oppositeends of said network for permitting the application of said controlpotential to said reversely connected tubes only while a relatively highlevel impulse passes from one end of said network to the other, andmeans connected between said reversely connected tubes and saidamplifier for maintaining a predetermined signal level at said amplifierwhile said relatively high level impulse is traversing said network.

9. The combination of a signal channel including an intermediatefrequency circuit, a leading audio amplifier, a signal conveying delaynetwork in said channel and connected with said ampliher to receivesignals therefrom, a circuit opening device and a following audioamplifier, and means including an over biased intermediate frequencyamplifier connected between said circuit and said device for operatingsaid device to open said channel while a relatively high level impulseis traversing said delay network.

10. A signal channel including a signal conveying delay network betweenthe input and output terminals of said circuit, a circuit control deviceconnected to an intermediate terminal of said network for receivingsignals therefrom, means including control tubes energized at theopposite ends of said network for controlling the operation of saiddevice in response to a relatively high level of the impulses suppliedto said channel, and means operable to modify said energization toproduce operation of said device over a predetermined range of signallevels.

11. A signal channel including a delay network, a circuitopen-circuiting device connected to an intermediate terminal of saidnetwork, means energized from different points of said network forcontrolling the operation of said device in response to a relativelyhigh level of the impulses supplied to said channel, and means operableto modify energy passing through said device in accordance with thecarrier level of said impulses.

12. The combination of a signal channel including an intermediatefrequency circuit, a leading audio amplifier, a signal conveying delaynetwork in said channel and connected with said amplifier to receivesignals therefrom, a circuit opening device and a following audioamplifier, means including an over-biased intermediate frequencyamplifier connected between said circuit and said device for operatingsaid device to open said channel, and means connected with the delaynetwork for causing said channel to remain open, while a relatively highlevel impulse is traversing said delay network.

13. In a signal amplifying channel, the combination of an intermediatefrequency circuit, a detector, a time-delay network, a circuit openingdevice, and a following audio frequency amplifier arranged in the ordernamed in signal-conveying series relation in said channel, and meansincluding an over-biased intermediate frequency amplifier connectedbetween said intermediate frequency circuit and said circuit openingdevice for operating said device, and means connected with the delaynetwork for effecting operation of said device for a period including ashort interval preceding the arrival of a high level impulse at saiddevice, the duration of said high level impulse at said device, and ashort interval following the presence of said high level impulse at saiddevice.

14. In a radio receiving system, the combination of means providing anaudio frequency detector, an audio frequency amplifier for amplifyingsignals received from said detector, and a noise impulse-interruptingcircuit interposed between said amplifier and detector means, comprisingan audio frequency signal delay network connected with said detectormeans to convey signals therefrom to said amplifier, means for derivingaudio frequency signals from said network intermediate its ends, saidlast-named means including a controllable signal conveying connectionwith said amplifier having a pair of diode biased amplifier tubes and acoupling capacitor between said tubes and said amplifier, the charge onsaid capacitor being controlled by said tubes, a pair of control tubescoupled to the delay network one at each end thereof, means foradjusting said tubes whereby they are actuated in response to noiseimpulses of predetermined high amplitude, and means for applying energyto the diode-biased amplifier tubes through said last-named tubes toactuate said diode-biased amplifier tubes when said control tubes areactuated by said impulses.

, W'INFIELD R. KOCH.

